circuit breaker operation _ electrical4u

9/9/2014 Circuit Breaker Operation | Electrical4u

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Electrical Objective Questions - MCQs

Circuit Breaker Operation

Circuit Breaker Opening Operation Requirement

Circuit Breaker Closing Operation Requirement

The primary function of an electrical circuit breaker is to provide opening and closing the electric current carrying

contacts. Although it is seems to be very simple. But we should remember, that, one circuit breaker remains at its closed

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position for maximum period of its life span. Very occasionally it is required to operate a circuit breaker for opening and

closing its contacts. Hence, circuit breaker operation must be very reliable without any delay or sluggishness. For

achieving this reliability the circuit breaker operating mechanism becomes more complex than it was rst thought.

Opening and closing distance, as well as stroke between contacts and velocity of moving contacts during operation,

the most important parameters to be considered during designing circuit breaker. Contact gap, traveling distance of

moving contacts and their velocity are determined by types of arc quenching medium, electric current and voltage rating

of a circuit breaker.

A typical circuit breaker operating characteristic curve is shown in the graph below.

Here in the graph, X axis represents time in milli seconds and y axis represents distance in milli meter.

Lets at time, T electric current starts owing through the closing coil. After time T the moving contact starts traveling

towards xed contact. At time T moving contact touches xed contact. At time T the moving contact reaches at its close

position. T - T is overloading period of these two contacts (moving and xed contact). After time T the moving contact

bounce back little bit and then again comes to its xed closed position, after time T .

Circuit Breaker Operating Characteristic

Now we come to the tripping operation. Lets at time T electric current starts owing through trip coil of the circuit

breaker. At time T moving contact starts traveling backward for opening the contacts. After time T , the moving contact

nally detaches the xed contact. Time (T - T ) is over lapping period. Now at time T the moving contact comes back to

its nal open position but here it will not be at rest position since there will be some mechanical oscillation of moving

contact before coming to its nal rest position. At time T the moving contact nally comes to its rest position.

Circuit Breaker Opening Operation Requirement

The circuit breaker is desired to be at open position as fast as possible. It is because of limiting contacts erosion and to

interrupt faulty electric current as rapidly as possible. But total travel distance of the moving contact is not determined

only by necessity of interruption of faulty current, but rather the contacts gap needed to withstand the normal dielectric

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stresses and lightning impulse voltage appears across the contacts when the CB is at open position.

The need for carrying the continuous electric current and for withstanding a period of arc in circuit breaker, make it

necessary to use two sets of contacts in parallel one the primary contact which is always made of high conductive

materials such as copper and the other is arcing contact, made of arc resistance materials such as tungsten or

molybdenum, which has much lower conductivity than primary contacts. During opening circuit breaker operation

the primary contacts open before the arcing contacts.

However, due to the dierence in the electrical resistance and the inductor of the electrical paths of the primary and

arcing contacts, a nite time is required to attain total electric current commutation, i.e. from primary or main contacts to

arcing contact branch.

So when the moving contact starts traveling from closed position to open position the contact gap gradually increases

and after some time a critical contact position reaches which indicates the minimum conduct gap required for preventing

re-arcing after very next electric current zero.

The remaining part of the travel is required only for maintaining sucient dielectric strength between contacts gap and

for deceleration purpose.

Circuit Breaker Closing Operation Requirement

During closing operation of circuit breaker the followings are required,

1. The moving contact must travel towards xed contact at sucient speed to prevent pre-arcing phenomenon. As

the contact gap reduces, arcing may start before contacts are closed nally.

2. During closing of contacts, the medium between contacts is replaced, hence sucient mechanical power to be

supplied during this circuit breaker operation to compress dielectric medium in the arcing chamber.

3. After hitting xed contact, the moving contact may bounce back, due to repulsive force which is not at all desirable.

Hence sucient mechanical energy to be supplied to overcome repulsive force due to closing operation on fault.

4. In spring - spring mechanism, generally tripping or opening spring is charged during closing operation. Hence

sucient mechanical energy also to be supplied to charge the opening spring.

circuit breaker operation _ electrical4u

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